1. Technical Field
The present invention relates to a technique for improving characteristics in representing gradation by turning on or off a pixel over subfields into which one field is divided.
2. Related Art
As is well known, a display panel has a display element, such as a liquid crystal element, for each pixel and changes a transmittance or reflectance of light for each pixel. In recent years, a projection display apparatus (projector) that emits light to such a display panel to generate a reduced image and projects an enlarged image of the reduced image using an optical system is becoming popular. One example of a technique for representing shades of gray in a display panel is a technique for dividing one field into a plurality of subfields, turning on or off a display element (pixel) for each of the subfields, changing a proportion of a period of time when the pixel is in an on (or off) state in one field, and thereby representing shades of gray (see, for example, JP-A-2007-148417).
With the above technique, gradation is represented in accordance with the proportion of a period of time when a pixel is in an on (or off) state in one field, so only two values for on and off levels are required for a driving voltage for a display element. Accordingly, the above technique can achieve a simpler structure by, for example, eliminating a digital-to-analog (D/A) converting circuit, in comparison with the traditional voltage modulation scheme.
A light source for emitting light to a display panel is AC-driven to achieve long life. When the light source is AC-driven, because the luminance of light emitted is changed, even with subfields having the same period, the subfields have different transmittances (reflectances) when a pixel is turned on or off. This causes an adverse effect on characteristics for representing gradation.